Along with development of sensor market and diversification of product types, occasions of recording of ambient environment, and detection of posture of exercise of human body and running condition of vehicles are increasing. Also, along with diversification of scenes of using sensors, occasions of using sensors under traditionally unexpected bad conditions of temperature environment, vibration noise environment, and electromagnetic noise environment are also increasing.
There are various types of sensors having movable structure including, for example, acceleration sensors and angular rate sensors (vibratory gyro) as widely known inertial sensors. In consideration of using two or more types of sensors, a hybrid inertial sensor provided as a package in which an angular rate sensor and an acceleration sensor are integrated has been proposed. For example, in Patent Documents 1 and 2, examples of hybrid inertial sensors in which an angular rate sensor and an acceleration sensor are combined are described. Since an angular rate sensor and an acceleration sensor are provided on the same substrate, a chip of a hybrid inertial sensor can be provided at a low cost, a high alignment accuracy between inertial sensors, and a small space occupancy.
The angular rate sensor includes a vibrating body and a Coriolis element provided inside the vibrating body. Coriolis force is generated when an angular rate is applied while the vibrating body is driven and vibrated at a constant frequency, and the Coriolis element provided inside the vibrating body is shifted in a direction perpendicular to a vibration direction of the vibrating body. By detecting an amount of displacement of the Coriolis element by the Coriolis force, an angular rate can be detected. Here, the faster the speed of the vibrating body in drive control, the larger the Coriolis force. Thus, to improve detection sensitivity of the angular rate sensor, it is necessary to vibrate the vibrating body of the angular rate sensor at a high frequency and large amplitude.
However, a microfabricated vibrating body formed by micromachining technology is largely affected by damping of the air. Since damping is force which is generated in proportion to the speed of the vibrating body, when vibrating the vibrating body in a high-speed state, i.e., in a state at high frequency and large amplitude under a high pressure, influence of damping by the air is increased and it becomes impossible to vibrate the vibrating body at a high frequency and large amplitude. As a result, the angular rate sensor's detection sensitivity is greatly deteriorated. That is, it is preferable to seal the angular rate sensor in an air tight manner at low pressure at which it is difficult to receive viscosity resistance of fluid. By sealing the angular rate sensor in an air tight manner at low pressure, it is possible to reduce influence of damping to the vibrating body. Thus, it is possible to vibrate the vibrating body of the angular rate sensor at a high frequency and large amplitude, and thus the detection sensitivity of angular rate is improved.
However, if the acceleration sensor integrally provided in the package with the angular rate sensor is arranged in the low pressure state, it becomes easier for a movable body of the acceleration sensor to be very prone to vibrate. Normally, when acceleration is not applied to the acceleration sensor, it is preferable for the movable body of the acceleration sensor to be in a resting state. However, when the movable body of the acceleration sensor is arranged in a low-pressure state, viscosity resistance of peripheral fluid cannot be sufficiently received and the movable body is sensitively displaced even by external vibration noise, and thus even when acceleration is not applied, the movable body is displaced. That is, so much sensitive reaction of the movable body of the acceleration sensor is not preferable in view of improving the detection sensitivity of the acceleration sensor.
Accordingly, as an inertial sensor including an angular rate sensor and an acceleration sensor, using configurations and a manufacturing method as described in Patent Documents 1 and 2 is thinkable.
Further, in the air-tight sealing package described in Patent Document 3, laser light is irradiated to a pressure adjusting member inside the package, so that pressure in the package is adjusted.